In the winter months - basically October to April - the UK sea conditions are often too rough for going to the offshore sites we all enjoy.
When this happens we tend to head for the quarries and lakes inland to allow practice and keeping skills polished.
Since I returned to the UK from living in Asia for 16 years, I have used the sites to dive all winter and keep my skills up to scratch. It's not great - but it's better than 6 months out of the water.
Conditions are often silty and if an open water course or a group of divers with poor trim and buoyancy are in the sites it can seriously deteriorate he visibility. In bad conditions viz can drop to 1-2 meters (3-6 feet).. sometimes worse - but it settles down fairly quickly to give 10 feet of viz.
As I live near the South Coast I tend to use the sites there.. I have no experience of the sites in the midlands and up North - I add them for those that travel to that region. The list is not comprehensive - but gives some options.
South/South West Inland Sites
Wraysbury - Outskirts of London
Horsea Island Dive Centre - South Coast Near Portsmouth
Vobster Quay - Bristol/Bath Area
National Dive Centre - Chepstow - Has real depth up to 80M/250Ft
Stoney Cove Dive Centre
North of England